Xerographic reproduction, be it charged area development (CAD) copier reproduction or discharged area development (DAD) printer reproduction, involves the application of toner to the latent image of a photoconductor as the photoconductor passes through a developer station. Toner within the developer station deposits on the photoconductor's latent image due to the electrical charge that is carried by the toner, and due to the electrostatic reproduction development field that exists between the photoconductor's latent image and the portion of the developer station on which a supply of toner is carried.
For example, in a copier, and some printers, the photoconductor may be initially charged to a relatively uniform, high magnitude, negative voltage. The charged photoconductor is then subjected to the influence of an imaging station, where the photoconductor is selectively discharged in only its image background area, thereby leaving a highly charged latent image on the photoconductor. The photoconductor is now transported through a developer station, such as a magnetic brush developer station. A magnetic brush developer station typically includes a rotating metal cylinder that carries a layer of positively charged toner powder and negatively charged magnetic carrier beads. This cylinder is typically connected to a source of development electrode reproduction bias voltage whose polarity, in this case, would be negative, and whose magnitude would be less negative than that of the photoconductor's latent image.
Within the teachings of the present invention, such a development electrode field may comprise an AC field, a DC field, or may include both AC and DC components.
As a result of this developing process, positive toner moves from within the developer station to deposit on the photoconductor's negative latent image. This action takes place under the influence of the negative reproduction development field that extends from the negative metal cylinder of the developer station to the more negative photoconductor latent image.
Optical density of the photoconductor's toned latent image, at a location downstream of the developer station, is dependent upon factors such as (1) the quantity of toner in the developer station (i.e., the toner concentration), and (2) the magnitude of the reproduction development field.
Since toner is consumed as reproductions are made, many schemes have been originated to automatically replenish the developer station's toner supply. One of these schemes is known in the art as a patch sensor system.
In this type of toner concentration sensor, the intensity of light that is reflected from a toned test patch is compared to a control or reference intensity value that represents an intensity indicative of proper toner concentration. When toner concentration is proper, these two values, i.e., the control value and the measured value, are generally equal. As toner is depleted during use of the reproduction device, the light reflected from the test patch area increases, and an error signal is provided to a toner dispensing mechanism, causing toner to be added to the developer station, thus bringing toner concentration back to the predefined control concentration.
A specific type of patch sensing toner concentration control apparatus provides that the signal representing actual toner concentration is generated by comparing (1) the light reflected from a bare photoconductor patch to (2) light that is reflected from the toned test patch. Generally, such a patch sensing toner concentration control apparatus compares the ratio of these two reflected light magnitudes to a control ratio, and determines if toner need be added to the developer station as a result of this comparison.
A patch sensing toner concentration control apparatus of this type is disclosed in U.S. Pat. Nos. 4,178,095, 4,179,213 and 4,183,657. Apparatus of this type is useful in the practice of the present invention, and these patents are hereby incorporated by reference.
A feature of the present invention provides a grey toned patch, such as is described in U.S. Pat. No. 4,466,731, also incorporated herein by reference.
Examples of prior attempts to control the optical density of reproduction output are: U.S. Pat. No. 4,432,634 provides for image density control by changing the voltage to which the photoconductor is charged, and maintains a control difference between this value and the reproduction bias voltage; U.S. Pat. No. 4,502,777 detects conditions that cause the characteristics of the photoconductor to vary, and utilizes the detected conditions to maintain photoconductor charge substantially constant; and U.S. Pat. No. 4,564,287 provides for measurement of the potential of a latent image, and the control of image forming conditions such as development bias voltage, photoconductor charge voltage and original exposure intensity as a result of this measurement.
Examples of prior patch sensing toner concentration control schemes are: U.S. Pat. No. 4,272,182 provides a patch sensor in which a predetermined, constant bias voltage is used to develop the patch area, and wherein variable bias voltage can be used while forming reproduction output in accordance with the desired density for the reproduction output; U.S. Pat. No. 4,279,498 provides a patch sensor wherein the operating condition of the sensor is compensated for as a function of changes in reproduction parameters, such as variation in charge potential, developing bias voltage and/or power supply voltage; U.S. Pat. No. 4,312,589 provides a patch sensor wherein photoconductor charging is controlled as a function of how long a non-use time period the reproduction device has experienced; U.S. Pat. Nos. 4,348,099 and 4,341,461 provide a patch sensing scheme having a precopy mode of operation that sets the initial development bias, and a subsequent copy mode of operation that step-adjusts development bias, to achieve the normal bias level, and thereafter controls toner dispensing to the developer station; U.S. Pat. No. 4,377,338 provides a patch sensor wherein a multi-shade test pattern is initially formed and reflectance values for this test pattern are stored in memory, with subsequent reflectance values being compared to these stored values in order to control parameters such as toner replenishment, illumination lamp energization levels, and bias voltage level; U.S. Pat. No. 4,522,481 provides a patch sensor wherein the bias voltage that is used when forming the patch is varied in accordance with a fatigue standard associated with the photoconductor; U.S. Pat. No. 4,533,234 provides for a comparison of the toner density on a patch area to a reference density, and operates to jointly control the addition of toner to a developer station and adjustment of the development bias at this station, in order to control the quality of reproduction output; U.S. Pat. No. 4,551,004 provides a patch sensor utilizing collimated light; U.S. Pat. No. 4,551,005 provides a constant size sensor pattern for both equal size and unequal size copying, this resulting in the same pattern cleaning load for each type of copying; and U.S. Pat. No. 4,572,654 provides a black optical mark and a white optical mark that are reflected onto the photoconductor to form two test patches whose toner density ratio is used to control the feeding of toner to a developer station.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.